1. Field of the Invention
The present invention relates to apparatus and methods for fabricating microelectronic packages. In particular, the present invention relates to low profile microelectronic packages, which are fabricated without a substrate.
2. State of the Art
Higher performance, reduced cost, increased miniaturization of integrated circuit components, and greater packaging densities of microelectronic devices are ongoing goals of the microelectronics industry. One method of increasing the density of microelectronic device packages is to stack the individual microelectronic dice within the packages. Furthermore, low-profile (thin) microelectronic packages are in high demand for use in small electronic devices, such as cell phones. Thus, the microelectronics industry fabricates a variety of thin, stacked packages.
FIG. 8 illustrates one such package known as an Ultra Thin Molded Matrix Array Package (UTMMAP). The UTMMAP 200 comprises a first microelectronic die 202 (such as a microprocessor, a chipset, a memory device, an ASIC, and the like) attached by its back surface 204 to a first surface 206 of a carrier substrate 208 (such as an interposer, a motherboard, a back surface of another microelectronic die, or the like). A first plurality of bond wires 212 extend from bond pads (not shown) on an active surface 214 of the first microelectronic die 202 to land pads (not shown) on the carrier substrate first surface 206 to make electrical contact therebetween, as will be understood by those skilled in the art. A second microelectronic die 216 is stacked by its back surface 218 on the first microelectronic die 202 with an appropriately sized spacing device 220 disposed therebetween to allow clearance for the first bond wires 212. The second microelectronic die 216 makes electrical contact with the carrier substrate 208 through a second plurality of bond wires 222 extending between bond pads (not shown) on an active surface 224 of the second microelectronic die 216 and land pads (not shown) on the carrier substrate 208. The stacked microelectronic dice are then encapsulated with a molding compound 226. The carrier substrate 208 also includes a plurality of external contacts 228 attached to a second surface 232 thereof. These external contacts 228 are used to connect the package to an external component (no shown), as will be understood to those skilled in the art.
Another thin, stacked package configuration is known as a Foiled Flex Ball Grid Array (FF-BGA). Such a package includes the use of a flexible substrate to route electrical traces from the second microelectronic dice stack to a position between the first microelectronic dice stack and the carrier substrate to make electrical contact therewith. FIG. 9 shows such an FF-BGA package 240, wherein a first microelectronic dice stack 242 and a second microelectronic dice stack 244 are attached to and in electrical contact with a first surface 248 of a flexible substrate 246 through wire bonds 252 and 254, respectively. The first microelectronic dice stack 242 and second microelectronic dice stack 244 may be fabricated in a manner discussed above with regard to the UTMMAP 200 of FIG. 8. A molding compound 256, 258 is dispersed proximate each of the first microelectronic dice stack 242 and the second microelectronic dice stack 244, respectively.
The flexible substrate 246 includes conductive traces (not shown) disposed therein, thereon, and/or therethrough, which make contact with an array 264 of external interconnects 266 (such as solder balls) disposed on a second surface 262 of the flexible substrate 246 proximate the first microelectronic dice stack 242. Thus, both the first microelectronic dice stack 242 and the second microelectronic die stack 244 have external interconnects 266 within the array 264. The flexible substrate 246 is bent such that a back surface 272 of the first microelectronic dice stack 242 can be attached to a back surface 274 of the second microelectronic dice stack 244 with a layer of adhesive 276. The external interconnects 266 are attached to a substrate 278 using a C4 (controlled collapse chip connect) process.
Although such approaches result in effective thin, stacked packages, these packages have poor thermal performance (i.e., do not dissipate heat well from the microelectronic dice therein) due to the poor thermal conductivity of the molding compound and carrier substrates used by these packages. Therefore, it would be advantageous to develop a thin, stacked package that is capable of effective thermal dissipation.